This invention relates to a voice band multiplex transmission system which permits talking on the telephone simultaneously with the transmission and reception of some other information using the same telephone circuit.
If it is possible to transmit and receive some information while speaking on the telephone, the function of the telephone can be expanded. There has been proposed a method in which when a character, figure or the like is written down by a pen on a tablet, the position of the pen at each moment is outputted in the form of analog or digital data of x and y co-ordinates and these two x and y data are received to reconstruct the original character, figure or the like. Since the character, figure or the like is written by hand, the x and y data are relatively low in speed. It would be very convenient if these x and y data could be transmitted and received via a telephone circuit while speaking on the telephone.
As systems for transmitting and receiving some information while speaking on the telephone, there have been proposed a system that transmits a voice and other information using different circuits and a system that frequency-divides or time-divides the same telephone circuit for transmitting a voice and information individually. Of these conventional systems, the system employing a plurality of independent circuits for transmission requires circuits of the same number as the information to be transmitted and received and hence is not preferred from the viewpoint of efficient utilization of circuit.
The frequency division system is a system that divides a transmission frequency band and transmits a voice signal and an information signal in the divided, different bands. Accordingly, the voice frequency band for voice transmission is narrower than an ordinary voice transmission frequency band and a portion of the voice frequency component is removed, resulting in deteriorated voice quality such as lowered loudness and intelligibility. For enhancement of the voice quality, there has been proposed a method that applies the transmitted voice to a non-linear circuit by which the voice component in the removed band is synthesized approximately. But it is doubtful to what extent the voice information once lost can be recovered by such processing. Also there has been proposed a method of compressing the information of a voice by the band compression techniques, but this method is still in the stage of study since various band compression techniques are confronted with a problem in the voice quality.
The time division system is a system that transmits a voice and other information while changing them over at short-time intervals and interpolates the voice after reception. In this case, the voice quality is deteriorated by cutting-off of the voice waveform and its discontinuous connection. Another method that has been proposed is to compress the voice waveform in terms of time, but this is still in the stage of study. Further, there has been proposed a method of inserting the information signal in a pause/silence period, but this method is not capable of completely simultaneous transmission and further presents a problem in that a voice switch is needed.
A system that superimposes the voice and the information on each other is proposed in I.B.M. Technical Disclosure Bulletin 1964, 4, "Voice-Data System". According to this system, a data signal is phase modulated and the modulated output is transmitted after being superimposed on the high-frequency portion of the voice. Since the high-frequency portion of the voice is usually smaller in energy than the low-frequency portion, the high-frequency portion of the voice is regarded as a noise with respect to the modulated data signal, and the data is received and demodulated and the data signal in the voice is removed therefrom utilizing the demodulated output. In practice, however, the high-frequency portion of the voice may sometimes have a relatively large amount of energy, and consequently the data signal cannot correctly be demodulated in some cases.
An object of the present invention is to provide a voice band multiplex transmission system which permits simultaneous transmission of a voice and some other information via the same voice circuit without partial removal of a voice component, and consequently with good volume and intelligibility and with substantially no deterioration of the voice quality.
Another object of the present invention is to provide a voice band multiplex transmission system which performs voice transmission of good quality without losing a portion of the voice signal, and permits correct demodulation of information transmitted simultaneously with a voice being signal and superimposed thereon.